1. Field of the Disclosure
The present disclosure relates to telemetry systems for communicating information from a downhole location to a surface location, and, more particularly, to a method of removing noise at the downhole location produced by downhole sources.
2. Description of the Related Art
Drilling fluid telemetry systems, generally referred to as mud pulse systems, are particularly adapted for telemetry of information from the bottom of a borehole to the surface of the earth during oil well drilling operations. The information telemetered often includes, but is not limited to, parameters of pressure, temperature, direction and deviation of the well bore. Other parameters include logging data such as resistivity of the various layers, sonic density, porosity, induction, self-potential and pressure gradients. This information is important to efficiency in the drilling operation and is used in reservoir development.
MWD Telemetry is required to link the downhole MWD components to the surface MWD components in real-time, and to handle most drilling related operations without breaking stride. The system to support this is quite complex, with both downhole and surface components that operate in step.
In any telemetry system there is a transmitter and a receiver. In MWD Telemetry the transmitter and receiver technologies are often different if information is being up-linked or down-linked. In up-linking, the transmitter is commonly referred to as the Mud-Pulser (or more simply “the pulser”) and is an MWD tool in the BHA that can generate pressure fluctuations in the mud stream. The surface receiver system comprises sensors that measure the pressure fluctuations and/or flow fluctuations, and signal processing modules that interpret these measurements.
Down-linking is achieved by either periodically varying the flow-rate of the mud in the system or by periodically varying the rotation rate of the drillstring. In the first case, the flow rate is controlled using a bypass-actuator and controller, and the signal is received in the downhole MWD system using a sensor that is affected by either flow or pressure. In the second case, the surface rotary speed is controlled manually, and the signal is received using a sensor that is affected.
For uplink telemetry, a suitable pulser is described in U.S. Pat. No. 6,626,253 to Hahn et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference. Described in Hahn '253 is an anti-plugging oscillating shear valve system for generating pressure fluctuations in a flowing drilling fluid. The system includes a stationary stator and an oscillating rotor, both with axial flow passages. The rotor oscillates in close proximity to the stator, at least partially blocking the flow through the stator and generating oscillating pressure pulses. The rotor passes through two zero speed positions during each cycle, facilitating rapid changes in signal phase, frequency, and/or amplitude facilitating enhanced data encoding.
Drilling systems (described below) include mud pumps for conveying drilling fluid into the drillstring and the borehole. Pressure waves from surface mud pumps produce considerable amounts of noise. The pump noise is the result of the motion of the mud pump pistons. The pressure waves from the mud pumps travel in the opposite direction from the uplink telemetry signal. Components of the noise waves from the surface mud pumps may be present in the frequency range used for transmission of the uplink telemetry signal and may even have a higher level than the received uplink signal, making correct detection of the received uplink signal very difficult. Additional sources of noise include the drilling motor and drill bit interaction with the formation. All these factors degrade the quality of the received uplink signal and make it difficult to recover the transmitted information.
There have been numerous attempts to find solutions for reducing interfering effects in MWD telemetry signals. See, for example, U.S. Pat. Nos. 3,747,059 and 3,716,830 to Garcia, U.S. Pat. No. 3,742,443 to Foster et al, U.S. Pat. No. 4,262,343 to Claycomb, U.S. Pat. No. 4,590,593 to Rodney, U.S. Pat. No. 4,642,800 to Umeda, U.S. Pat. No. 5,146,433 to Kosmala et al, U.S. Pat. No. 4,715,022 to Yeo, U.S. Pat. No. 4,692,911 to Scherbatskoy, U.S. Pat. No. 5,969,638 to Chin, GB 2361789 to Tennent et al., and U.S. patent application Ser. No. 11/311,196 of Reckmann et al. These are examples of what are called “passive” systems in which measurements are made at or near the surface of the borehole to estimate and cancel pump noise and other surface sources of noise. Such passive methods cannot distinguish between the uplink telemetry signal and drilling-generated noise in the mud channel.
The present disclosure is a method and apparatus for active cancellation of noise, such as drilling noise, that is generated downhole.